Senator McCarthy's campaign to expose and eliminate Communists caused a great deal of alarm and outrage in the United States. While many citizens resented his tactics and did not agree that Communism was the dire threat he painted it to be, others were swept up in his passionate rhetoric and believed that the country was in danger from the spread of Communism. McCarthy believed that operatives and spies were present in many industries, particularly in Hollywood, and also within the government itself. The "enemies within" he mentions is a reference to those who are sympathizers who might eventually become full-blown Communists, and therefore enemies of the state, and he believed many people were in danger of being indoctrinated even if they merely attended a meeting.
The parallel of this idea in The Crucible is well represented. Arthur Miller uses the structure of the play and the context of the Salem witch trials to paint a dramatic portrait of the fear and paranoia present during the era of McCarthy's activities. The "enemies within" were God-fearing Puritans like Rebecca Nurse and Martha Corey, and it was believed even the pious were not safe from being bewitched. The notion that the Devil was able to turn good people into his servants was not only believed by many of the people in Salem Village in the 17th century, but is an apt metaphor for McCarthy's insistence that Communism could infect anywhere, and that even "good" Americans could become subjugated to its "evil."